Hydraulic pump, hydraulic system and hydraulic components usually adopt the structure of cylindrical slide valve. Theoretically, the valve core and valve body should be complete and aligned. Therefore, no matter how much pressure it is under, the force needed to move the valve core only needs to overcome the viscous friction, and the value should be very small (0.5 ~ 5N). But this is not the case in practice, especially in the medium and high pressure system, When the spool stops moving for a period of time (usually about 5min), occasionally the resistance can be as large as several hundred newtons, which makes it very difficult to move the spool. This is the so-called hydraulic clamping sign of the slide valve.

　　The hydraulic clamping in the hydraulic system increases the wear of the slide valve and reduces the service life of the hydraulic components. In the control system, the displacement of valve core is usually controlled by electromagnet and spring with less pneumatic force. Hydraulic clamping will make the new action ineffective or unable to take action, resulting in adverse consequences of system operation. For example, the hydraulic valve spool in the continuation of high-pressure things, there is a sign of reset lag or unable to reset after pressure relief. Electromagnetic directional valve is also due to hydraulic clamping and switching slow, so that it can not change the sign.